There will probably be several issues with your performance for the first few past papers you take. My initial attempts were usually terrible. This puts a lot of students off, but there is actually nothing to worry about. When you do past papers the right way, your rate of improvement will be extremely high. This is because doing past papers is a form of active preparation.
One major problem that many students have is not having a clear idea of their own strengths and weaknesses. Remember that you will progress more quickly by working on your weaknesses. If you don't know your weaknesses, then you won't know what to focus on. This prevents you from working efficiently.
The only way to fully reveal your strengths and weaknesses is to take information away from yourself and then test to see if you can reproduce that information on your own. Essentially, you must recreate exam conditions in order to test your exam performance. Past papers and other forms of active preparation do exactly this. You are forced to retrieve and convey your knowledge on demand, just like in a real exam. Your weaknesses are immediately made clear because it is very obvious when you can't answer a question.
Writing notes or reading a textbook won't test you in the same way you because all of the information is in front of you. They are forms of passive preparation. Nothing is forcing you to think, which makes it extremely easy to drift into a state of disengagement. If you aren't engaged, you aren't being challenged and you won't learn quickly. There is no instant feedback, so you have no clear indication whether you are making progress. With passive preparation, you are leaving your learning entirely to chance.
After doing each past paper and fully writing out corrections, you will have produced a set of model answers. Several past papers completed in this manner will serve as a truly excellent study material. Much better than traditional revision notes. When you learn from a set of model answers to questions that you have attempted, you are learning in an extremely exam-centric way because you are learning with the best possible context.
Remember: it is not just what you know, but how you learn what you know that affects the way you use that knowledge in an exam. When you learn from past papers, there is no need for you to translate your knowledge to an answer in the real exam. Your knowledge will already be in exactly the right form. Your approach to writing answers will develop into the style that gets the highest exam scores.
With past papers, your learning is targeted towards a specific purpose. You will learn material as a set of responses to questions rather than as part of the aimless sort of absorption that most students waste their time with.